1. Field of the Invention
The present invention is directed to a method for correcting artifacts in magnetic resonance images, the artifacts being of the type caused in a magnetic resonance device by transverse magnetic field components that are oriented transverse to the basic magnetic field, the magnetic resonance device having gradient coils for generating magnetic gradient fields and a high frequency transmitter for exciting magnetic resonance signals.
2. Description of the Prior Art
A method for this purpose is disclosed in the article by Robert M. Weisskoff, Mark S. Cohan, Richard R. Rzedzian with the title xe2x80x9cNon-axial Whole-Body Instant Imagingxe2x80x9d, published in the Journal of Magnetic Resonance in Medicine, Vol. 29, 1993, pp. 796-803, in connection with echo planar imaging (EPI). This article notes that, for imaging a slice that lies apart from a central plane, secondary magnetic gradient fields can cause image distortions and ghosts in these images. The secondary gradientfields are always associated with the imaging gradient fields and can be derived directly from the Maxwell equations. These additional, undesired gradient fields are therefore also known as Maxwell terms. The article notes, for example, that a transverse z-gradient in the x-component of the magnetic field is always associated with an imaging x-gradient. For reducing image distortion caused by the Maxwell terms, it is proposed to add an additional read-gradient between the high frequency-excitation pulse and the 180xc2x0 refocusing pulse. Thus, prior to the refocusing pulse, the same phase is added to the excited spins, like after the refocusing pulse, by the Maxwell term. In the correction by means of adding gradient pulses into the measurement sequences, it is a disadvantage that this method is only even possible for a few specific sequence types, and that, under circumstances, the performance capability of the measurement sequences is decreased by additional switching times for gradients. On the other hand, other optimizing criteria in the sequence design are precluded by the boundary conditions for the addition of gradient pulses.
The article also mentions that the image distortions caused by the Maxwell terms can be corrected by a Z2-shim coil using a compensation current. This is, however, characterized in the article as being complicated because the compensation current must have a square-shaped curve given sinusoidal gradients. Moreover, the correction is incomplete because the gradient coils produce no Z2 field.
Finally, the article also mentions that the artifacts caused by the Maxwell terms can be corrected in post-processing of the image. The post-processing is similar to the compensation of irregularly shaped, imaging gradients, in which the actual field is taken into account in the correction algorithm. A disadvantage of distortion correction in the post processing is that the signal losses by destructive noise of several echos cannot be compensated.
A method for the correction of image artifacts produced by Maxwell-terms given echo-planar-imaging is disclosed in German OS 198 21 780. Therein, the frequency errors and phase errors caused by the Maxwell-terms are calculated and subsequently compensated during data collection by means of a dynamic adjustment of the receiver frequency and receiver phase.
U.S. Pat. No. 5,877,629 discloses a method for the correction of artifacts in MR-images, whereby previously calculated phase errors are used in order to determine currents for driving coils for the compensation of Maxwell fields of higher orders. Alternatively, the reference frequency in the receiver correspondingly changes during the reception.
An object of the present invention is to provide a method for correcting artifacts in magnetic resonance images that are generated by transverse magnetic field components , wherein the above-discussed disadvantages of conventional methods can be substantially avoided.
This object is inventively achieved in a method wherein, during a measurement sequence for a correction area, a frequency correction value is fed to the high frequency transmitter, the frequency correction value being determined dependent on the transverse magnetic field components. In general, additional magnetic fields in the magnetic resonance device effect a shifting of the resonant frequency and thus, among other things, a distortion in the imaging. Using the present invention, it is proposed to compensate the shifting of the resonant frequency (that is caused by transverse magnetic field components) with the aid of an appropriate resetting of the excitation frequency. Since the Maxwell term exhibits a location (spatial) dependency, this method requires the specification of a defined spatial reference point, in the proximity of which the correction of artifacts is complete. Correction in an area around this reference point also can still be attained, if the location dependency of the frequency shift is represented as a series development of the transverse field components. In the inventive method, the 0-th order term of this series development, corresponding to a location-independent frequency shift, is compensated by means of an appropriate resetting of the high frequency excitation frequency. The 1st order term that corresponds to a spatial, linear, location dependent frequency shift, is compensated by a gradient offset in the corresponding axis or axes. Terms of higher orders can be taken into consideration in principle as well, e.g. by a suitable dynamic drive of the coils. This is not specifically discussed herein, however, because such higher order terms usually do not significantly contribute to the aforementioned artefacts, and therefore correction of such terms is usually not worthwhile.
By means of the inventive method, a spatial translation of the behavior of the Maxwell terms can be achieved. This applies in the axial as well as in the radial directions. The xe2x80x9caxial directionxe2x80x9d means the direction of the basic magnetic field and xe2x80x9cradial directionxe2x80x9d means the transverse direction relative thereto. Given magnetic resonance devices of conventional solenoid construction, for example, a translation of the behavior of the Maxwell terms could be used in the axial direction for a nearly artifact free measurement with large slice displacement. A displacement of the behavior of the Maxwell terms in the transverse direction could help avoid artifacts, for example, is an optimized examination of a shoulder.
Application of this method to multi-slice techniques is possible in so far as a slice can be allocated to each gradient pulse, on the signals of this slice being acted on by the gradient pulse.
In an embodiment, the frequency correction value is selected dependent on a symmetry parameter, this symmetry parameter being characteristic for a symmetrical behavior of the transverse magnetic field components. Thus, the behavior of the Maxwell terms given asymmetrical gradient coils can be shifted in space such that it corresponds to that of a symmetrical gradient coil.